1. Field of the Invention
The present invention relates to electronic devices each having at least one organic material based insulating layer, in particular, electronic devices, each having at least one organic material based insulating layer, excellent in weather resistance and reliability, in spite that a portion of the insulating layer is exposed on the surface of the device, and also having easiness in fabrication and reworkability.
2. Description of the Related Art
As electronic instruments such as personal computers and cellular phones are reduced in size, electronic parts such as coils and capacitors mounted in the interior of electronic instruments are required to be reduced in size or to be formed in thin films.
For example, a thin film magnetic head includes a multi-layer lamination of thin film layers such as magnetic films and insulating layers, and further includes coil conductors formed therein. A thin film inductor also has a structure in which a substrate is provided, on the outer surface thereof, with electrodes to be connected to external circuits, and a conductor to work as a self-inductance is provided inside the substrate in a manner covered with insulating layers.
As the insulating layers in such a thin film magnetic head and such a thin film inductor, insulating layers made of inorganic materials such as SiO2 and insulating layers made of organic materials such as novolac resin and polyimide resin have hitherto been known (Japanese Patent Laid-Open No. 2000-114045). Among these insulating layers, insulating layers made of organic materials are used particularly for applications to thin film magnetic heads and thin film inductors in which heat treatment at high temperatures affects the specific resistances of magnetic layers, because by use of organic materials, such insulating layers are able to be made at relatively low temperatures and thick insulating layers are able to be made easily.
For example, in Japanese Patent Laid-Open No. 7-296329, a first insulating layer made of polyimide and a second insulating layer made of a novolac resin are used in a laminated manner. This case discloses that the use of the polyimide insulating layer having a high volume resistivity permits attaining a higher improvement of the insulating properties than a single use of the novolac resin; and the novolac resin participating in lamination reduces a risk of the break of the coil conductor in the peripheral areas of the insulating layers as compared to the case where the polyimide resin is used alone because the shape of the cured film made of the novolac resin is smooth. Also in this case, a coil conductor is formed on the laminated insulating layer to fabricate a thin film magnetic head.
The metal constituting the coil conductor is a relatively easily ionizable metal material such as copper (Cu). Consequently, when a photosensitive polyimide is used for an insulating layer covering the coil conductor, unless the coil conductor made of Cu is covered with Ni or the like and then the covering insulating layer is formed, there occurs a problem that Cu ions penetrate into the photosensitive polyimide to form a Cu complex with the polyimide. The formation of such a Cu complex causes development failure in the development after exposure to prevent obtaining a desired pattern. In particular, such development failure causes a problem when patterns such as contact holes are formed on the polyimide layer covering a conductor.
Also, in the course of fabrication, when a desired pattern is not obtained due to a pattern failure, a procedure of rework in which the failed resin pattern is peeled off and the resin concerned is again applied to form the pattern is adopted as the case may be; herein involved is a problem that polyimide is excellent in weather resistance and accordingly poor in reworkability.
As disclosed in the above Japanese Patent Laid-Open No. 7-296329, the use of a laminate structure composed of a polyimide layer and a novolac resin layer permits attaining a certain level of reliability improvement due to polyimide and a certain level of workability due to a novolac resin. However, the present inventor has found that if a novolac resin is exposed on the surface of a device, the weather resistance of the device is drastically degraded.